10.1 Non-Condensable Gases and Ingress

Overview of Non-Condensable Gases (NCGs)

In industrial ammonia refrigeration systems, maintaining peak thermodynamic efficiency and safety is paramount. One of the most common and costly detractors from system performance is the presence of non-condensable gases (NCGs). Non-condensables are gases that do not transition from a vapor state to a liquid state at the temperatures and pressures typically found in the high-pressure side (condenser and receiver) of a refrigeration system.

Unlike anhydrous ammonia (R-717), which condenses from a vapor to a liquid at common condensing temperatures (e.g., 86°F to 96°F) under corresponding saturation pressures (e.g., 154.5 psig to 184.2 psig), NCGs remain strictly in their gaseous state. The physical properties of these gases are vastly different from ammonia, as detailed in the table below:

Because these gases cannot condense, they do not mix liquid-to-liquid with ammonia. Instead, they occupy valuable volume within the high-pressure side of the system, reducing the active surface area of the condenser and driving up system pressures and operating costs.

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Thermodynamic Principles: Dalton's Law of Partial Pressures

To understand how NCGs affect system pressure, we must look to Dalton's Law of Partial Pressures. Dalton's Law states that the total pressure exerted by a mixture of non-reacting gases in a vessel is equal to the sum of the partial pressures of the individual gases that make up the mixture.

In a clean ammonia system, the pressure in the condenser is solely determined by the temperature of the condensing liquid (saturation pressure). However, when NCGs are present, the pressure gauge on the condenser will register a total pressure ($P_{\text{total}}$) that is the sum of the saturated ammonia pressure ($P_{\text{ammonia}}$) and the partial pressure of the non-condensable gases ($P_{\text{NCGs}}$):

$P_{\text{total}} = P_{\text{ammonia}} + P_{\text{NCGs}}$

Diagnostic Calculation: Checking for NCGs

Operators must regularly check for the presence of NCGs by comparing the actual system pressure to the theoretical saturation pressure corresponding to the temperature of the liquid refrigerant leaving the condenser.

For an accurate reading, the temperature should be measured at the liquid drain leg of the condenser or inside the high-pressure receiver, where the liquid is in a saturated state.

Step-by-Step Worked Example:

1. Measure Condenser Liquid Temperature: A technician measures the temperature of the liquid line leaving the evaporative condenser and finds it is 90.0°F.
2. Find Saturated Pressure of Ammonia: Using an official Ammonia Saturation (Pressure-Temperature) Chart, the saturation pressure of pure R-717 at 90.0°F is determined to be 165.9 psig.
3. Read Actual Discharge Pressure Gauge: The high-stage compressor discharge gauge (or condenser inlet gauge) reads 181.9 psig.
4. Calculate Excess Pressure ($P_{\text{NCGs}}$): $P_{\text{NCGs}} = P_{\text{total}} - P_{\text{ammonia}}$ $P_{\text{NCGs}} = 181.9\text{ psig} - 165.9\text{ psig} = 16.0\text{ psig}$
5. Conclusion: The system is operating with 16.0 psi of excess pressure due to the presence of non-condensables. This indicates a clear need for purging.

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Sources of NCG Ingress

Non-condensable gases do not generate spontaneously in large quantities under normal, clean operating conditions. Instead, they enter the system through three main pathways:

1. Operation Under Vacuum (Low-Side Pressures)

In many industrial refrigeration applications—such as blast freezers, ice cream freezers, or low-temperature cold storage—evaporator operating temperatures can be extremely low (e.g., -40°F or lower).

• At -40°F, the saturation pressure of ammonia is 8.7 inches of mercury vacuum (in. Hg vac), which is approximately 10.4 psia.
• Since the pressure inside the low-side piping is below atmospheric pressure (14.7 psia), any breach in system sealing will draw atmospheric air (nitrogen, oxygen, water vapor) inward.
• Common entry points include:
  • Compressor Shaft Seals: Especially on older open-drive reciprocating compressors.
  • Threaded Connections and Flanges: Microscopic gaps in gaskets under temperature cycling.
  • Valve Stem Packing: Worn packing glands that seal under positive pressure but fail under vacuum.
  • Piping Corrosion: Pinhole leaks caused by rust under insulation (CUI).

2. Maintenance and Servicing Activities

Whenever a portion of the system is opened for maintenance (e.g., replacing a valve, cleaning an oil strainer, rebuilding a compressor, or replacing an evaporator fan motor), air enters the piping.

• Prior to returning the serviced equipment to service, it must be evacuated using a vacuum pump to remove all air and moisture.
• Inadequate Evacuation: If the technician fails to pull a deep enough vacuum (typically required to hold below 500 microns or 29.92 in. Hg depending on IIAR/company standards) or rushes the evacuation process, air and moisture will remain trapped in the isolated section. Once the isolation valves are reopened, this trapped air is swept into the high-pressure side.

3. Oil Thermal Breakdown and Chemical Reactions

• Oil Cracking: In high-compression-ratio applications, compressor discharge temperatures can exceed 300°F (149°C) if cooling systems or intercoolers fail. Under these extreme temperatures, the refrigeration oil (especially mineral-based oils) can undergo thermal cracking, breaking down into carbon sludge and non-condensable hydrocarbon vapors, including methane and hydrogen gas.
• Chemical Reactions: If water vapor (moisture) enters the system, it reacts with ammonia to form ammonium hydroxide. Under high pressures and temperatures, this mixture can react with internal metals to generate hydrogen gas ($H_2$). Hydrogen, being extremely light and having an ultra-low boiling point (-423°F), acts as a highly volatile non-condensable gas.

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Operational Penalties and Effects of NCGs

The consequences of failing to purge NCGs go far beyond simple gauge readings; they directly impact the facility's bottom line and the physical integrity of the equipment.

1. Increased Discharge Pressure (Head Pressure)

Because NCGs collect at the top of the condenser and receiver where gas velocities are low, they reduce the volume available for active condensation. The compressor must discharge vapor against this elevated total pressure.

2. Increased Compressor Power Consumption

An elevated discharge pressure increases the compression ratio (discharge pressure divided by suction pressure). The compressor motor must perform more work to compress the refrigerant vapor to this higher pressure.

• Rule of Thumb: For every 4 psi of excess discharge pressure, compressor motor energy consumption increases by approximately 2%.
• In a large plant utilizing a 500-horsepower compressor, a 20-psi excess pressure penalty (causing a 10% increase in electricity demand) can increase annual operating costs by tens of thousands of dollars.

3. Reduced System Refrigeration Capacity

• Higher discharge pressures reduce the volumetric efficiency of the compressor, meaning the compressor pumps less mass flow of refrigerant per stroke or rotor revolution.
• Rule of Thumb: For every 4 psi of excess discharge pressure, system refrigeration capacity decreases by approximately 1%. The system must run longer to achieve the same cooling effect.

4. Impaired Condenser Heat Transfer

In the condenser, ammonia vapor must contact the cold tube surface to reject heat and condense into liquid. NCGs are swept along by the ammonia gas stream toward the condensing surface. Because the NCGs cannot condense, they form a stagnant gaseous blanket or boundary layer around the condenser tubes. This gas layer acts as an effective thermal insulator, significantly reducing the overall heat transfer coefficient (U-factor). The condenser becomes less effective, further driving up the required condensing temperature and head pressure to reject the same heat load.

5. Chemical Degradation, Sludge, and Wear

• Oil Oxidation: The oxygen introduced via air ingress reacts with the refrigeration oil at high discharge temperatures. This results in oil oxidation, which forms varnish, acids, and thick black carbon sludge.
• Bearing Failure: Sludge blocks oil filters, oil separators, and small lubricating passages, starving compressor bearings and causing catastrophic mechanical failure.
• Corrosion: Water vapor reacts with ammonia to form ammonium hydroxide ($NH_4OH$), an alkaline solution that attacks system metals and contributes to copper plating on bearing surfaces.

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Troubleshooting and Preventative Action

To manage and prevent NCG issues, operators should adopt the following procedures:

1. Keep Logs: Record daily discharge pressures and liquid temperatures. Calculate the excess pressure offset weekly.
2. Prioritize Vacuum Testing: Never rely on "purging with refrigerant" to clear air after maintenance. Always use a high-vacuum pump and verify with a calibrated micron gauge.
3. Monitor Vacuum-Stage Suction Lines: Perform routine bubble-jar checks on systems that operate below 0 psig to verify that air is not leaking into the low-temperature side.
4. Control Discharge Temperatures: Keep compressor discharge temperatures below 250°F (121°C) to prevent oil cracking and hydrogen generation.